build: replace `echo -e` with `printf`

build: replace `echo -e` with `printf`

`echo -e` is not portable and we can end up with:

      GEN      matches.man
    -e      + ./libxt_addrtype.man
    -e      + ./libip6t_ah.man
    -e      + ./libipt_ah.man
    -e      + ./libxt_bpf.man
    -e      + ./libxt_cgroup.man
    -e      + ./libxt_cluster.man
    -e      + ./libxt_comment.man
    -e      + ./libxt_connbytes.man
    -e      + ./libxt_connlabel.man
    -e      + ./libxt_connlimit.man
    -e      + ./libxt_connmark.man
    -e      + ./libxt_conntrack.man
    [...]

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Phil Sutter <phil@nwl.cc>